The peripheral B cell neoplasm chronic lymphoid leukemia/small lymphocytic lymphoma represents the most common lymphoid leukemia. As the name implies, presentation can be as either leukemia or lymphoma. However, the two presentations of this neoplasm, chronic lymphocytic leukemia (CLL) and small lymphocytic lymphoma (SLL), are morphologically, phenotypically, and genotypically indistinguishable, differing only in the degree of peripheral blood lymphocytosis.
CLL is the most common leukemia of adults in the Western world. In CLL, the peripheral blood contains small, round lymphocytes with scant cytoplasm. Involvement of the bone marrow is observed in all cases of CLL and most cases of SLL, taking the form of interstitial infiltrates or nonparatubular aggregates of small lymphocytes. The tumor cells in CLL and SLL express the pan B cell markers CD29 and CD20. In addition, CD5—a T cell marker that is expressed only on a small subset of normal B cells—is present on the tumor cells. The immunophenotype of CLL cells is unique. CLL cells co-express the B lymphocyte lineage marker CD19 and the T lymphocyte marker CD5. CLL cells also exhibit a characteristic level of expression of immunoglobulin receptor. Tumor cells typically also have low-level surface expression of Ig heavy chain, with either kappa or lambda light chains.
CLL is a clonal malignancy of B lymphocytes. The disease is usually indolent, with slowly progressive accumulation of long-lived small lymphocytes that are immunoincompetent and respond poorly to antigenic stimulation. CLL is incurable with conventional cytotoxic chemotherapy (Cheson et al., Blood 1996; 87:4990-4997; and Keating et al., Blood 1993; 81:2878-2884). The hallmark of CLL is isolated lymphocytosis. The white blood cell count is usually greater than 20,000/μL and may be markedly elevated to several hundred thousand. The diagnostic requirement for CLL is an absolute lymphocyte count of greater than 4000/mm3. CLL is manifested clinically by immunosuppression, bone marrow failure, and organ infiltration with lymphocytes. Immunodeficiency is also related to inadequate antibody production by the abnormal B cells. With advanced disease, CLL may cause damage by direct tissue infiltration.
In some cases, patients may develop cutaneous lymphoma deposits—erythomatous lesions on the skin. The lesions may contain an atypical lymphoid dermal infiltrate of small, round B cells.